Unpackaged or bare semiconductor dice are being increasingly used in the manufacture of electronic devices that employ multi-chip modules (MCM). These unpackaged dice are mounted directly to a substrate such as a printed circuit board. With unpackaged dice, semiconductor manufacturers are required to supply dice that have been tested and certified as known good die. Known good die (KGD) is a collective term that denotes unpackaged die having the same quality and reliability as the equivalent packaged product.
For testing an unpackaged die, an electrical pathway must be established between the unpackaged die and external test circuitry. An interconnect is typically used to form a temporary electrical connection with the die. The interconnect includes the contact structure that physically contacts and forms an electrical connection with contact locations on the die.
These contact locations are typically die pads (e.g., bond pads or test pads) located on the face of the die in electrical communication with integrated circuitry. Die pads are relatively small in size and are closely packed on the face of the die. As an example, bond pads are typically polygonal in shape (e.g., square, triangular) and are only about 100 .mu.m on a side. The spacing between bond pads is on the order of 50 to 100 .mu.m or less. The contact structure for an interconnect must therefore be precisely formed to accommodate the size and spacing of the bond pads.
In addition, the contact structure on the interconnect preferably forms a low-resistance ohmic electrical connection with the contact location. An ohmic electrical connection is one in which the voltage appearing across the connection is proportional to the current flowing for both directions of current flow. In order to form a low-resistance ohmic connection, a contact structure must penetrate an oxide layer covering the bond pad. An aluminum bond pad for instance, will typically include a native oxide layer which forms during the manufacturing process. This oxide layer, which may be about 100.ANG. or more in thickness, must be either penetrated or wiped away to establish an electrical connection that is ohmic. At the same time, however, damage to the bond pad must be kept to a minimum. A bond pad may only be about 1.mu. thick and is thus relatively easy to damage.
One well known contact structure used for forming interconnects and other connection systems is the needle probe. Needle probes however, exhibit a variety of shortcomings. In particular, needle probes are susceptible to damage, misalignment and rapid loss of contact force. In addition, needle probes may not provide uniform contact to vertically misaligned pads and may damage the pads. Needle probes also require extensive maintenance and adjustment. All of these problems are compounded by the higher integration of semiconductor dice and a corresponding decrease in the size and pitch of bond pads. This has led to the recent development of other connection systems for testing semiconductor dice at both the wafer level and the die level.
Some recently developed connection systems include compliant contact structures adapted to flex to accommodate variations in the planarity of the die pads. As an example, U.S. Pat. No. 5,264,787 to Worth et al. describes an interconnect that includes an array of metal plated contacts formed on a flexible membrane. This type of contact is manufactured by Packard-Hughes under the trademark Gold Dot.TM.. U.S. Pat. Nos. 5,103,557 and 5,323,035 to Leedy disclose other contact structures that are mounted on a flexible substrate or membrane. In addition, U.S. Pat. No. 5,207,585 to Byrnes et al. discloses an interconnect that includes a flexible interface pellicle having electrodes for contacting raised conductive bumps on a die.
Although some prior art interconnects compensate for variations in-the vertical location of pads on a die, there can still be problems in piercing an oxide layer covering the contact to form an electrical connection. Some interconnects utilize an arrangement in which the flexible membrane for mounting the contacts is tensioned prior to engagement with the die. When the tension is released, the contacts move across the die pads and scrub away the oxide layer to establish an electrical connection. Such a scrubbing action however increases the complexity of the interconnect and may damage the die pads.
One recently developed technique for preventing damage to a die pad, while establishing an electrical connection, is to limit the penetration depth of a contact into the die pad. U.S. Pat. No. 5,326,428 to Farnworth et al., the present applicant, discloses such a penetration limited contact structure. With such a contact structure, raised silicon contact members are formed on a silicon substrate. Each raised contact member includes a raised projection, such as a point or knife edge, that penetrates the die pad to establish an electrical connection. The penetration depth however, is limited by the dimensions of the projection and by a stop plane provided by a top surface of the contact member.
The present invention is directed to an improved contact structure that also includes a self limiting feature. The contact structure is especially useful for forming interconnects and other connection systems for semiconductor dice. The improved contact structure is mounted on a compliant substrate and is adapted to penetrate a contact location on a semiconductor die to a limited penetration depth to establish an electrical connection.